This disclosure relates generally to superconducting magnets, and more particularly, to a superconducting magnet having a very high field-to-current ratio. The disclosure also relates to a method of producing the superconducting magnets, as well as uses thereof.
Superconducting magnets are those magnets fabricated, or selected winding materials that, when operated at cryogenic temperatures, exhibit essentially zero resistance to the flow of current. Accordingly, magnetic fields can be produced and maintained with considerably less power than required for magnets of conventional construction.
Whenever any portion (or all) of a superconducting magnet is quenched, i.e. becomes resistive rather than superconducting, a large amount of energy must be dissipated. This released energy can create extreme heating conditions at the point where such a quench initiates. The heating problem is normally addressed by preventing localized concentration of the heat coupled with a transfer of heat to the mandrel and heat sink.
Current superconducting magnets for producing strong magnetic fields, such as four (4) Tesla or more, require strong currents, such as ten (10) amperes or more. Often, even greater currents in the range of several hundred amperes can be required for superconducting magnets. Even the lowest of these currents requires a sizable current source and sizable power cables leading to the magnet, thus increasing the heat load on the cryogenic system. Moreover, the size and weight of current superconducting magnets tend to increase in relation to the higher magnetic field strengths desired.
There are many applications for a superconducting magnet where even a minimum of these conditions are detrimental to the efficiency of the system. For example, it is desired that superconducting magnets for space applications use much smaller currents without sacrificing the achievable magnetic fields. This would significantly reduce the size of power supplies as well as reduce the load on any cryogenic system needed to achieve the required lowered temperatures for superconductivity. Moreover, for many applications and space applications in particular, it is desirable for the system to be as compact and/or lightweight as possible. An Adiabatic Demagnetization Refrigerator (ADR) system is one such example.
An ADR system produces cooling (or heating) by the interaction of a refrigerant material in the bore of a superconducting magnet. The magnetic field of the superconducting magnet is ramped up causing the material to heat. Likewise, ramping down the magnetic field down causes it to cool. In some applications, the superconducting magnet is employed in an environment close to absolute zero. A conventional ADR system is a “single-shot” ADR. In this type of ADR system the refrigerant material continues to be magnetized, generating heat, which flows to a heat sink. This continues until full field is reached. At full magnetic field, a heat switch is deactivated and the refrigerant material is thermally isolated from the heat sink. The refrigerant material is then demagnetized to cool it to the desired operating temperature. In general, the refrigerant material will then be receiving heat from components parts. The heat is absorbed and operating temperature maintained by slowly demagnetizing the refrigerant at a predetermined rate. Heat can continue to be absorbed until the magnetic field is reduced to zero, at which point the ADR has run out of cooling capacity.
Over the last few years there has been a growing need for more advanced ADR cooling technology. The space industry has been a pioneer in this technology because ADRs are the only low temperature (below 0.2° K) refrigeration technology that does not use any fluids, and therefore does not have the design constraints imposed by gravity. The trend in developing ADRs is toward using continuously operating multiple cooling stages, as this arrangement allows for greater efficiency by reducing parasitic heat flows within the refrigerator, and greater operating temperature range. In this process, each stage requires a superconducting magnet. In particular for space applications, each stage of the continuously operating ADR systems requires smaller, lighter-weight superconducting magnets than are currently available.
Accordingly, there is a need for smaller, lighter-weight superconducting magnets capable of achieving high magnetic field strengths at low currents.